A tilt rotor aircraft is one which typically has a pair of tiltable pods containing an engine and rotor system supported on 15 aircraft wings. The pods are movable between a vertical position when the engine driven rotors serve as helicopter rotors for vertical take off and landing, and a horizontal position where the rotors serve as propellers for forward flight.
Typically, tilt rotor aircraft in operation today utilize a fixed diameter rotor system that represents a distinct design compromise between the requirements for hover and the requirements for high speed cruise. Because the rotor thrust in hover must lift the entire gross weight, the rotors are grossly over sized for cruise a flight regime, where the wings carry the weight and the rotor thrust required for propulsion is only a small fraction of the gross weight. Conversely, the rotors are undersized for the helicopter mode, a compromise to the cruise mode size requirement, and this results in an undesirable high disk loading in hover. Disk loading trends for conventional tilt rotors are on the order of twice that for conventional helicopters for a given gross weight, resulting in high hover power consumption, reduced hover endurance, and excessive downwash velocities.
Lift efficiency (lift per unit of engine power) decreases as disk loadings are increased. Most helicopters operate at disk loadings that are less than 10 psf to achieve reasonable lift efficiency and to maintain acceptable downwash velocity in the rotor wake. Disk loadings of about 14 psf or higher result in downwash velocities of hurricane proportion. Present tilt rotor aircraft have disk loadings in the range of about 15-25 psf. For a tilt rotor, downwash velocities are further accelerated in the plane of symmetry along the fuselage due to the interference effects of side by side rotor arrangements. Another disadvantage of high disk loadings is that the autorotation characteristic in a helicopter mode is badly degraded. The autorotative rate of descent will be high and the stored rotor kinetic energy available to cushion touch down will be relatively low. Thus, while helicopters can land safely without power, it is unlikely that conventional tilt rotors will be able to do so.
The preceding section describes how a large rotor diameter is an advantage for operating the aircraft in the helicopter mode as it provides for low disk loading which results in efficient operation, low noise levels and diminished downwash velocities. On the other hand, a relatively small diameter is an advantage in the propeller mode to reduce tip speed and blade area for improved propulsive efficiency, minimized blade aero-elastic response to the air loads encountered and for simplified ground handling. One method for accommodating these conflicting requirements is to use a variable diameter rotor.
By utilizing a variable diameter rotor, the rotor can be operated at maximum diameter in hover for efficiency and low disk loading operation, and then the diameter can be decreased by any amount up to about 40% for high speed cruise in the airplane mode, reducing both tip speed and blade area. Thrust capability is much more nearly balanced to the cruise requirements with no need for the large rpm reduction required by conventional tilt rotors.
Telescoping variable diameter blades are those with two or more rigid segments per blade that can be telescoped with respect to each other to vary rotor diameter. Retraction ratios vary depending upon the number of telescoping segments. The simplest configuration, with two radial segments, can achieve a retraction ratio of approximately 1.8 to 1. A variety of mechanisms have been proposed to retract these blades, including cables and straps, screw drives, compressible and incompressible hydraulic systems, rack and piston gearing, plus various forms of rigid mechanical rods. Telescoping has the advantage that it allows retraction at full rotational speed without introducing significant bending loads in any direction. Reduction of rotor radius with a two segment telescoping blade is more than adequate to accommodate high speed tilt rotor aircraft.
Various mechanisms have been proposed for providing variable diameter rotors. For example, U.S. Pat. Nos. 4,142,697, 3,884,594 and 3,713,751 disclose telescoping outer blade portions which are movable over inner blade sections. However, while providing variable diameter capability, such designs have incorporated compromises in that, to accommodate the extension and retraction of one blade section over another, the blade twist must be linear throughout its entire length. Linear twist blades are not optimally efficient as, at high thrust coefficient/solidity ratios, the inboard airfoil section experiences a premature stall condition in hover flight. Also, in forward flight, large regions of negative lift are created on the inboard blade sections, which decreases cruise efficiency.